iOS (Jailbroken): AirFloat is a jailbreak app that turns your iOS device into an AirPlay receiver so you can wirelessly stream music to it from anywhere.

AirFloat is incredibly simple. Just launch the app, and suddenly your iOS device is an AirPlay receiver that can recieve music from iTunes, another iPhone, or anything else that support AirPlay. It's a simple little app, but if you have an old iOS device not getting much use (or you just have an extra that you want to stream audio to), AirFloat is a free app that works right out of the box. AirFloat only handles audio, but if you're looking for something with a little more power, AirServer is a $4.99 app that handles video and photos as well.